In the field of construction engineering, it is known from the prior art that cylindrical holes, in particular blind holes, having a typical diameter of 10 mm to over 25 mm, are drilled in concrete or stone by means of conventional drills, in particular concrete drills, and dowels, reinforcing irons and concrete anchors etc. are anchored in these holes by means of hardening substances, in particular adhesive agents. A problem in the prior art consists in that the inner surfaces of a hole drilled in concrete or stone is comparatively smooth, and the adhesive agents, e.g. adhesive, liquid concrete, etc. does not have an adequate hold, or does not have a sufficiently large interaction surface, on such a smooth inner surface of a drilled hole. This results inherently in a potential lack of robustness, in particular in respect of axial tensile forces.
In order to improve the hold of components, such as support elements, hooks, reinforcements and the like, that are to be adhesive-bonded, for example, in drilled holes and, in particular, in order that they can be inserted with improved robustness against axial tensile forces, it is therefore necessary to enlarge the interaction surface between the hole wall and the adhesive agent. This can be effected by roughening the surface, and in particular by producing transverse grooves in a drilled hole.
Undercutting tools that convert an axial force to a radial force by means of a spreading mechanism are known from the prior art, for the purpose of producing transverse grooves in a previously created drilled hole. Cutting elements are pressed radially against the wall of the drilled hole by the radial force, such that, upon rotation, transverse grooves, or undercuts, can be produced in the wall of the drilled hole.
There are known in this case, firstly, solutions in which an undercutting tool is supported at the end of a blind hole, i.e. on the bottom of the hole, for the purpose of defining a working position, as well as, secondly, solutions in which the undercutting tool is supported on the surface surrounding the hole to be worked.
A solution of the first type is disclosed, for example, in DE 196 09 798. The drilling tool described therein comprises a cylindrical receiving body having a non-round guide channel, passage openings that are open radially outward, two cutting elements that can be displaced outward radially through these passage openings, and a slide having a non-round cross-section. The slide projects at least partially into the guide channel of the receiving body, goes through through-bores of the cutting elements, is displaceable, against the force of a spring, against the direction of drilling, and serves to displace the cutting elements radially. A plurality of successively disposed guide regions of the slide have a guide edge, stop edges, and slide surfaces that taper against the direction of drilling, as well as slide surfaces that widen against the direction of drilling, the cutting elements being able to slide along such slide surfaces. The slide is supported on a base of a drilled blind hole.
When the receiving body is displaced in the drilling direction relative to the slide support at the end of the blind hole, sliding of cutting elements along a slide surface results in the cutting element being displaced radially out of an initial position, in which the cutting elements do not project over the outer contour of the receiving body, into a working position, which projects over the outer contour of the receiving body and in which rotation of the tool causes transverse grooves to be cut into the drilled hole.
The drilling tool according to DE 196 09 798 is thus based on converting an axial force to a radial force, the slide, having a non-round cross-section, having the function of a spreading element. However, the manner in which the slide and the cutting elements are made to act together is disadvantageous in several respects: on the one hand, owing to the through-bores of the cutting elements, the cutting elements have a certain amount of play for the slide. This play can easily lead to the cutting elements becoming caught on the wall of a drilled hole, as a result deviating from the intended, precisely radial alignment when subjected to load. On the other hand, when the cutting elements are being guided back after the axially acting force has been removed, damage can occur, for example axially extending cutting or grinding marks in the drilled hole, as a result of a merely slow-acting return movements. In addition, the described conversion of axial force to radial force is relative inefficient, since the slide surfaces taper or widen gradually in the direction of drilling, but extend virtually parallel to the direction of drilling over larger regions in which, although the action of centrifugal force does cause the cutting elements to be moved radially, axial force is nevertheless not actively converted to radial force. Since the cutting elements are designed with through openings for the slide, they also have a relatively low level of robustness, such that they can easily break when subjected to load.
A technical solution of the second type, i.e. with the undercutting tool being supported on the surface surrounding the drilled hole to be worked, is disclosed in DE 29 28 555. This document describes an undercutting tool having cutting elements that can be displaced in the radial direction, by means of an axially displaceable spreading element, by axial force upon a rotatable tool shank. The spreading element is realized as a spindle having thrust surfaces realized so as to run obliquely in the axial direction. The cutting elements have oblique surfaces, running in the axial direction, which can slide along the thrust surfaces of the spindle-type spreading element. The axially movable spreading element is surrounded by a drilling sleeve, which is immovable in the axial direction and which, in the region of the cutting elements, has recesses, or openings, in which the cutting elements are held substantially without axial play, but so as to be movable in the radial direction.
The cutting elements, which, with their oblique surfaces, slide along the thrust surfaces of the spreading element, and which have outwardly facing cutting edges, are driven radially outward as a result of movement of the spindle-type spreading element in the axial direction, such that, when in the driven-out state, they can then produce an undercut in the drilled hole.
This solution also has various disadvantages, which are likewise caused by the functionality of spreading element and cutting element being split into two discrete components. Both the spindle-type spreading element and the cutting elements, which, with their axially aligned oblique surfaces, can slide on the thrust surfaces of the spreading element, are subject to a high degree of mechanical stress, since this type of interaction results in an axial force being only incompletely converted to a radial force and being converted, to a considerable extent, to a frictional force between these surfaces. In addition, in this case likewise, when the cutting elements are being guided back, by drawing the spreading element back axially; inertia or a slight tilting can result in damage to the tool or to the wall of the drilled hole to be worked.
What the known solutions of the prior art have in common is that the axially distributed cutting elements cannot move radially independently of one another. If the radial mobility of one cutting element is blocked, for example because the wall of the drilled hole is harder in one region than in another region—in particular because of a reinforcement iron or a discontinuity in the material in the wall of the drilled hole, in the case of the solutions of the prior art the radial mobility of the remaining cutting elements, which are not blocked per se, is also prevented.
The object of the present invention is to provide an improved undercutting tool, by which the described technical disadvantages of known tools of the generic type can be avoided. A constituent object consists in a more efficient conversion of axial force to radial force. A further constituent object is to prevent the cutting elements from becoming caught on the wall of the drilled hole. A further constituent object is constituted by rapidly and faultlessly guiding the cutting elements back out of their working position into their neutral position when the acting axial force ceases. A further constituent object consists in that the separating elements are able to separate independently of one another, and in that the radial blocking of one cutting element, for example caused by a reinforcement iron projecting into the drilled hole, does not result in the rest of the cutting elements being impeded in their cutting function. In addition, according to a further constituent object, the undercutting tool is to be such that servicing is made easier and worn cutting elements can be replaced in a rapid and simple manner when required.